THE pH of the surface ocean is a sensitive function of its alkalinity and total inorganic carbon concentration, properties which also control the partial pressure of atmospheric carbon dioxide17. Thus, an accurate proxy for past ocean pH could yield information about variations in atmospheric CO 2. Recently, it has been suggested that the boron isotopic composition of foraminiferal tests depends on the pH of sea water as well as its isotopic composition1,2. Here we present boron isotope and elemental data for sedimentary pore fluids and isotope data for bulk foraminiferal samples from a deep-sea sediment core. The composition of the pore waters implies that sea water boron concentrations and isotopic composition have been constant during the past 21 Myr, allowing us to reconstruct past ocean pH directly from the foraminiferal isotope data. We find that 21 Myr ago, surface ocean pH was only 7.4 ±0.2, but it then increased to 8.2 ±0.2 (roughly the present value) about 7.5 Myr ago. This is consistent with suggestions3-5 that atmospheric CO2 concentrations may have been much higher 21 Myr ago than today.
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